This invention relates generally to the recovery of cadmium and other metals from phosphoric acid and other acidic to slightly basic carrier solutions, and more particularly, to the use of an organophosphene extractant which selectively extracts the desired metal species.
Valuable minerals are often recovered from ores by partial dissolution and subsequent processing to a final product. An example of the approach is the dissolution of apatite, a complex calcium phosphate mineral, in sulphuric acid to produce "wet process" phosphoric acid. The phosphoric acid may then be purified and concentrated, and subsequently processed into phosphates suitable for use in fertilizer and industrial applications.
When the ore is dissolved in acid, metal species present in minor amounts are dissolved as impurities into the phosphoric acid, and the impurities can be recovered from the acid solution to avoid contamination of the final product or, where feasible, for their economic value. Because the concentrations of the metallic impurities are ordinarily low, their recovery from acid solution is usually accomplished in two steps, a concentration step and a final purification step. The technologies such as electrowinning and crystallization used in the final purification step are relatively well known, and the present invention is directed toward the concentration step.
In the solvent extraction process for concentrating metal values, the solution containing the metal values is contacted to an organic extractant dissolved in a water-immiscible organic solvent. The solvent containing the metal-laden extractant is then separated from the aqueous solution for subsequent recovery of the metal from the extractant as a concentrated aqueous solution.
Solvent extraction processes for concentrating metal values typically suffer from several shortcomings, including low efficiency and a lack of selectivity of the metal value extracted. The low efficiency of extraction from the aqueous solution leaves metallic impurities in the solution which may be detrimental to the final product, and also results in a reduced economic benefit for extraction of the metal values. A second significant problem is the lack of selectivity of typical extractants, so that several different metal species may be extracted at one time, necessitating a costly separation step in subsequent purification of the metal values. Additionally, in some instances it is desirable that the metal values be physically separable from the aqueous solution without the need for an immiscible organic solvent, and such a procedure is not possible with liquid-liquid extraction. Accordingly, there has been a need for an improved approach to the concentration and recovery of metal values from dilute acid and other carrier solutions. The present invention fulfills this need, and further provides related advantages.